Ziegler-Natta catalysts comprising titanium compounds and organo-aluminum compounds are generally used to obtain olefin polymers in the olefin polymerization field. For example, in CN85100997A and CN1453298A, it recites using a catalyst in the preparation of polypropylene to obtain a propylene polymer, wherein said catalyst comprises a solid catalyst component, an organo-aluminum compound as a cocatalyst component and an organo-silicon external electron donor compound as an improving agent for stereospecificity, and said solid catalyst component is mainly composed of titanium, magnesium, chlorine and an internal electron donor compound. Currently, the research for catalysts mainly focuses on the following aspects: enhancing the polymerization activity of catalysts, enhancing stereospecificity of polyolefins, improving hydrogen regulation sensitivity of catalysts, improving particle shape of olefin polymers, reducing the residual Cl in the polymers, and so on.
When a supported catalyst component with a dialkoxy magnesium carrier is used, the obtained propylene polymer has the advantages of a good particle shape, small fine powder content and good stereospecificity. In order to obtain a catalyst component with well performance for olefin polymerization, a dialkoxy magnesium carrier with well performance should be first prepared.
There are many researches into using a dialkoxy magnesium carrier to prepare a catalyst component for olefin polymerization. EP0459009 discloses a catalyst component for olefin polymerization, and the preparation thereof comprising dispersing diethoxy magnesium in an alkylbenzene to form a suspension, contacting said suspension with titanium tetrachloride and a phthalyl dichloride at 80-125° C., washing the resultant by an alkylbenzene, and finally obtaining a titanium-containing catalyst component. The catalyst obtained from said catalyst component has high polymerization activity and long-lasting activity, but the obtained propylene polymer has a lower bulk density.
EP 0811639 mainly discloses a solid catalyst component for olefin polymerization, which is prepared by reacting a titanium halide, an aromatic dicarboxylic ester with an alkoxy magnesium. By controlling the indexes such as the bulk density and the average particle shape and controlling the heating rate from the first contacting temperature to the reacting temperature of the titanium halide and the alkoxy magnesium (the heating rate is controlled within 0.5-20° C./min), a solid catalyst component is obtained, and then a polyolefin with high isotacticity and bulk density can be obtained, but he obtained polymer has high fine powder content.
CN 101054424A discloses that, the prepared dialkoxy magnesium is suspended in toluene, and titanium tetrachloride is added after decreasing the temperature of the suspension to 0° C. The temperature of the suspension is risen, and an internal electron donor is added. Toluene is added for washing after full reaction, then toluene and titanium compounds are added for treating, and finally hexane is used for washing. The obtained solid catalyst component has high polymerization activity and good sphercity degree, but the particular size distribution thereof is not concentrated.
In a solid catalyst component for olefin polymerization with magnesium, titanium and internal electron donors as the basic components, an internal electron donor compound is essential for obtaining a polymer with high stereospecificity. The commonly used electron donor compounds disclosed are multi-carboxylic esters. For example, phthalates are disclosed in many patents, the specific compounds being di-n-butyl phthalate, and di-iso-butyl phthalate. When a solid catalyst component obtained from such compounds is used with a cocatalyst component and an external electron donor to form a catalyst, and when said catalyst is used for olefin polymerization, it has high polymerization activity and good polymer stereospecificity, but the obtained polymer has disatisfactory wide molecular weight distribution and poor processability.